The ferry has been assigned to the Edmonds-Kingston route and was slated to go back into service soon on that route.
The incident happened while the 188-car vessel was in for routine, regularly scheduled maintenance work Sunday, spokeswoman Marta Coursey said.
A failure in one of the drive motors left parts of the ferry near the engine melted and charred.
The Walla Walla had been out of service for painting and other routine maintenance for about three months at the time of the accident, Coursey said.
It will likely be out of service for at least the next few months while crews work to repair the vessel, Coursey said. The drive motor that was damaged is one of four motors that turn the propellers.
Immediately following the incident, the ferry system started an internal investigation per standard protocol, according to Coursey. Preliminary findings indicate that there was no explosion.
"This is a serious incident and we're going to do a very thorough and detailed investigation into what happened," state ferries chief David Moseley said in a written statement. "We hope to return the vessel to service as soon as possible."
A third-party contractor, the Cadick Corporation, is conducting the investigation. The company will be assisted by the state ferries' senior port engineers, safety systems managers, and electrical engineers. The state also has reached out to the Department of Labor and Industries and the U.S. Coast Guard for assistance to ensure that all employee safety requirements were met, Coursey said. Detailed results of the investigation are expected within two to three weeks.
In an effort to repair the Walla Walla, the state is working with the engine manufacturer to see if it's possible to use a spare drive motor the state already owns and has in a warehouse. If the spare motor will work, ferry officials hope to move it to the manufacturer's shop by the end of next week.
Vessels will be switched between routes to maintain service while the Walla Walla is unavailable.
Bill Sheets: 425-339-3439; firstname.lastname@example.org.
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